It is known to provide vehicles such as lawn tractors with front wheel steering. These conventional lawn tractors typically include a pair of rear wheels that engage the ground and are driven by the lawn tractor's power source or internal combustion engine for propelling the lawn tractor across the ground. A pair of front ground engaging wheels are also provided which are typically not driven by the engine. The front wheels are pivotable to steer the vehicle for executing turns during forward and reverse vehicle travel. The front wheels are operatively coupled via a linkage to a steering wheel located in the vehicle operator station. The operator seated on a seat within the operator station grasps the steering wheel with his hands and turns the wheel in the direction of the desired turn. The linkage transmits the motion of the steering wheel to the front wheels and thereby executes a turn in the desired direction.
When the operator wishes to stop the vehicle from turning when traveling forwardly he can release the steering wheel, and the front wheels will tend to straighten out and assume positions corresponding to straight forward travel. The operator may wish to guide the steering wheel back to its straight forward position in order to control how quickly the vehicle comes out of the turn and straightens out. Since the wheels tend to straighten out by themselves after execution of a turn during forward travel, it is relatively easy for the operator to return the vehicle to its straight forward travel mode. The tendency of the front wheels to return to their straight forward orientation after a turn helps the operator return the steering wheel to its straight forward mode.
Some lawn tractors provide mechanical all-wheel-steering mechanisms, which serve to turn all of the vehicle's three or four ground engaging wheels to execute a turn. These vehicles have the advantage of being capable of relatively tight, small radius turns. This tight turning radius is desirable when using the lawn tractor to mow close around obstacles such as trees, posts or similar obstructions. All-wheel-steering mechanisms typically include a steering shaft extending downwardly from the vehicle. This steering shaft is operatively coupled with an arm that swings forward or backward as the steering shaft pivots in response to the steering wheel being turned by the operator. First and second link rods are coupled with the arm and extend fore and aft therefrom. The first and second link rods are operatively coupled with respective pivot plates between the front and rear pairs of wheels. Tie rods extend outwardly from each side of the pivot plates and are coupled with respective steering arms which control the angular orientation of each ground engaging wheel. As the steering shaft pivots, the outer end of the arm will shift fore or aft, which shifts the first and second link rods therewith. Shifting of the first and second link rods causes the front and rear pivot plates to swing to one side, which in turn shifts the tie rods, causing each wheel to turn in the proper direction during execution of a turn.
As described above, the front wheels' engagement with the ground during execution of a turn tend to apply a force which urges the front wheels back toward their straight forward orientation when the operator releases the steering wheel after execution of a turn during forward travel. However, the rear wheels' engagement with the ground tends not to urge the rear wheels back toward their straight forward orientation. Rather, engagement with the ground during turns in the forward direction of travel applies a force to the rear wheels that can tend to hinder the rear wheels from returning to their straight forward orientation. The rear wheels are linked with the front wheels via the steering linkage, and the force that tends to keep the rear wheels turned can counteract or overcome the force that tends to return the front wheels to the straight forward position. Therefore, it can be difficult for an operator to return the steering wheel and ground engaging wheels to the straight forward orientation after execution of a turn in the forward direction. This phenomenon can be particularly acute when the operator turns the steering wheel to its extreme turning positions corresponding with relatively tight turning radii. The operator may have difficulty returning the steering wheel to its straight forward mode. Furthermore, the operator may be unfamiliar with the handling of these all-wheel-steer lawn tractors since they tend to behave somewhat differently than front wheel steer lawn tractors.
It would therefore be desirable to provide a mechanical all-wheel-steer vehicle that is easily returned to straight forward travel when coming out of a turn. It would be desirable for such a steering mechanism to allow an operator to more easily return the steering wheel to its straight forward orientation when the operator wishes to again drive the vehicle straight forward after execution of a turn. It would also be desirable to provide a mechanical all-wheel-steering mechanism that handles more like a front wheel steering linkage than do conventional all-wheel-steering devices.